Pipelines for transporting fluids such as gas, oil or other combustible liquids are widely used. Trade and transport of combustible fluids is an economically important market with a large number of participants on the market. The amount of the combustible fluids being transported through the pipelines or being in storages is therefore a valuable parameter for statistics and other economic parameters. Data on how much of the fluid is transported through the pipelines can traditionally only be measured by companies having direct access to the inside of the pipelines. The companies having such access often do not share or provide this data to third parties or to the public.
Therefore methods and systems have been developed to measure the volumetric flow or flow rates in pipelines from the outside without having direct access to the inside of the pipelines or without having physical contact with the pipelines.
Documents WO 2005/042984 and WO 2008/030844 teach the use of acoustic sound transducers for producing and measuring acoustic signals emanating from the pipeline. The acoustic signals emanating from the pipeline often comprise a number of acoustic signals from different sources that are not always easy to separate from one another. The acoustic signals of interest have to be carefully filtered which involve advanced filtering techniques and a large amount of computing power. The acoustic signals will also be influenced and changed by changes in the environments.
U.S. Pat. No. 5,571,974 discloses a system for measuring the flow of coal particles in a flow string by measuring the vibration of a tube through which the coal particles flow. This method is not applicable for liquids that do not contain any particles.
German Patent DE 198 58 307 teaches an apparatus and a method for measuring the flow through a pipeline wherein a heat flow sensor is attached to the pipeline.
U.S. Pat. No. 4,984,460 describes a heat flow meter with two heat sensitive coils that wound around a conduit.
U.S. Pat. No. 6,883,369 B1 describes a heat flow sensor wherein a heat probe and a reference sensor are in direct contact with a tube.
U.S. Pat. No. 4,255,968 describes a method and an apparatus for measuring a flow through a tube by heating the tube and measuring a temperature difference directly on the tube.
It is an object to the present invention to provide a method and an apparatus for measuring a flow rate and volume of a fluid in a vessel without necessarily having direct access to the vessel.